Not only does the impedance drop below 4 ohms for most of the lowermidrange and upper bass, with a minimum value of 2 ohms at 86Hz, but there is also an amplifier-unfriendly combination of 3 ohms and –43° phase angle at 61Hz.From what I read about the REL and the way you have them connected, there is no xover, and both the REL subs and Sasha woofers are active and operating independently (if this is incorrect please comment because I would like to know myself).
I'm just wondering if using a different amp will give you more bass/mid bass/lower midrange instead of relying on additional subs from what you have. I have a feeling a different amp will help you more than additional subs will. I also think no matter what amp you have, the subs you have already will help regardless, especially for the 2ohms at 86HZ dip.
Might be worth something to pursue if you could borrow an amp that doesn't have issues going down to 2ohms (friend or dealer). I think if your willing to pay for shipping you could even try a Pass Labs from Reno Hifi (I'm not recommending Pass Labs, just as a way to test the theory).